Surface acoustic wave (SAW) devices have been widely used in various fields as signal filters for electric and electronic equipment that utilizes electromagnetic waves. For example, the SAW devices are applied to frequency filters for video signals processed in television sets and video tape recorders. Generally, the SAW device has a SAW chip formed by a piezoelectric substrate made of a piezoelectric material such as LiNbO3 or LiTaO3 on which comb electrodes for exciting surface acoustic waves are formed. The SAW chip is housed in a package to form the SAW device. The surface acoustic waves are propagated on the surface of the piezoelectric substrate. Thus, a protection film is not provided on the piezoelectric substrate and the comb electrodes, and a cavity is formed thereon. The SAW chip is sealed and protected.
For cost reduction of the SAW device, the SAW chip may be housed in a package that is made of resin and has a cavity. More particularly, the SAW chip is mounted on a base made of resin, and a cap is bonded to the base so as to seal the SAW chip. Thus, the SAW chip is sealed in the cavity defined by the base and the cap.
Japanese Patent Application Publication No. 2001-94389 discloses a SAW device in which convex and concave portions are provided in the bonding interface between the base and the cap for the purpose of improving the bonding strength.
Japanese Patent Application Publication No. 2008-131152 discloses a SAW device in which the bonding interface between the base and the cap is subjected to surface texturing or the like in order to improve the bonding strength.
The base and the cap are bonded by an adhesive applied to an outer portion of the base. The adhesive goes around the outer portion of the base, and hermetic sealing realized by the base and the cap is improved. In a process of mounting the SAW device on a printed-circuit board or the like, a heat history such as reflow is applied to the SAW device, and the inside of the package thermally expands. Thus, the base or the cap may pop on the printed-circuit board, and there is a difficulty in placing the SAW device in position. The base or the cap may be cracked and destroyed in the worst case.